Conventional flexible disk drives which operate with flexible disks having relatively low track density, e.g. 48 to 135 tracks per inch (tpi), typically do not have a control system to position the read/write head over the data tracks or to maintain it over the centerline of the data tracks during read or write operations. Rather, such disk drives operate "open loop" by use of a head positioning motor, such as a calibrated stepper motor, to correctly position the head over the data tracks. In order to substantially increase the track density in flexible disk drives, it is necessary to incorporate a servo control system to center and maintain the head over the tracks during read or write operations. While the need for a servo control system exists in any disk file with a relatively high track density, it is especially critical in the case of flexible disks because such disks are subject to nonuniform distortion due to temperature and humidity changes, spindle motor runout and other effects.
One well known method of incorporating servo information in a rigid disk drive is to use pre-recorded servo signals on equally angularly spaced sectors which extend out radially from the disk center. As the rigid disk rotates, the head receives sampled track position signals as the servo sectors pass beneath the head. This technique, or similar techniques which utilize prerecorded servo signals, is not suitable for use with flexible disk drives because flexible disks are generally available in unrecorded form and any recording of servo information in angularly spaced sectors would significantly increase the cost of the disks. Furthermore, the time required to record servo information in multiple sectors on each track would be prohibitive if recorded by the user's disk drive.
There are several conventional techniques for generating a track position error signal (PES) from the servo information within the servo sectors in rigid disk drives. In one technique, as disclosed in U.S. Pat. No. 3,812,533 to Kimura, et al., the servo signals for the data tracks are angularly staggered so that the time between a reference pulse and the servo signal for the particular track identifies that track within a group of tracks. In a second technique, as described in an article by R. K. Oswald entitled "Design of a Disk File Head-Positioning Servo" in the IBM Journal of Research and Development, November, 1974, pp 506-512, the PES is generated by comparing the amplitudes of the signals from servo blocks spaced on opposite sides of the track centerlines.
In addition to the incompatibility of servo sectors with flexible disk drives, neither of the rigid disk drive PES generating techniques is suitable for use with flexible disk drives because of the complex, relatively costly timing or analog amplitude detection circuitry required.
U.S. Pat. No. 4,188,646 to Sordello, et al. discloses a sector servo pattern for a recording disk in which adjacent servo tracks abut the centerline of the data track, the servo information in each servo track comprising a specific, unique frequency. The head senses the different frequencies from adjacent servo tracks and combines them with modulation frequencies from a phase locked oscillator. The resulting combined signal is used to correct the head position to locate it over the track centerline.
U.S. Pat. No. 4,346,413 to Hack discloses a servo control system for a flexible disk drive in which the servo information is a constant frequency slanted pattern within the data track, the slant having been formed by the erasure of a portion of the servo signal. The slanted erased portion thus creates a servo pattern of constant frequency in the radial direction. The amount of servo signal read by the head is a function of radial position.
U.S. Pat. No. 4,348,703 to Janosi discloses a servo system for a flexible disk drive and includes means for writing servo information on the flexible disk. The servo information is similar to that in the U.S. Pat. No. 4,188,646 in that it includes a specific frequency for each data track and means for combining the frequency patterns of adjacent servo tracks to position the head. The servo information is written in only one of the sectors of the flexible disk, the remaining sectors being dedicated to data.
U.S. Pat. No. 4,135,217 to Jacques, et al. discloses a disk drive which utilizes stored disk distortion or runout information to provide track profile information and U.S. Pat. No. 4,396,959 to Harrison, et al. discloses, at column 9, lines 31-53, a calibration technique for generating track profile information.